The present invention relates to rotation sensors, such as gyroscopes, and particularly to a fiber optic rotation sensor.
Fiber optic rotation sensors are particularly advantageous, since they operate without moving parts, and therefore, are potentially very reliable. Typically, rotation sensing is accomplished by detection of a rotationally induced phase shift, commonly referred to as the "Sagnac phase shift", between light waves which propogate in opposite directions around a closed loop formed by a coil of optical fiber. This "Sagnac" phase shift is defined by the well known relationship: ##EQU1## Where A is the area bounded by the loop of optical fibers, N is the number of turns of the optical fiber about the area A, omega (.OMEGA.) is the angular velocity of the loop about an axis which is perpendicular to the plane of the loop, and lamda (.lambda.) and c are the free space values of the wave length and velocity, respectively, of the light applied to the loop.
The present state of the art of fiber optic rotation sensors uses polarized light through single mode optical fibers. Early development efforts were characterized by non-reciprocal operation of the rotation sensors, due to the fact that single mode fibers have two orthogonal polarization modes, each of which generally propogates light at a velocity different than the other. The residual birefringence of the single mode fiber caused some of this polarized light to be cross-coupled into the other of the two polarization modes. Consequently, because of the different velocities of propogation in the two polarization modes, a non-rotationally induced phase shift was introduced, thereby causing non-reciprocal operation of the rotation sensors. It was later found that reciprocal operation could be achieved by placing a polarizer at the output side of the rotation sensor to block any light that was cross-coupled to the other polarization mode. Other solutions involved use of a polarization controller, either separately or in combination with the output polarizer, as disclosed in U.S. Pat. No. 4,410,275; issued Oct. 18, 1983, Entitled "Fiber Optic Rotation Sensor".